1. Field of the Invention
The present invention relates to a leakage detecting device for detecting a leak current of a display device with a plurality of light emitting diodes (LED), etc. and generating an alarm (display) signal.
2. Description of the Related Art
Generally, a large number of lighting equipments using a plurality of LEDs as a light source have appeared on the market. There are, for example, a rope type alarm illumination device for notifying of a construction site on the road side and a display device for displaying letters, pictures and signs, etc. by using a decoration display device incorporating a large number of LED for decorating buildings and shops in place of conventional neon signs and, furthermore, there are lighting equipments using LEDs as a light source recently as typified by a traffic light.
These lighting equipments are used not only indoors but often used outdoors, so that when processing in electric connections is weak, leakage may be caused due to an influence by humidity and dust.
Other than the above, an insulation defect caused by deterioration over time due to long time use may cause leakage, and damage by animals may cause leakage.
Next, a specific example of a key part of an LED display device using LED will be explained below. A configuration example of a line light source 100 produced by mounting a plurality of LEDs on a printed substrate will be explained. As shown in FIG. 1A, a wiring pattern is formed on a printed circuit board made by a glass epoxy, etc., a plurality of LEDs are arranged at predetermined intervals thereon and, furthermore, a power source IC, resistor, capacitance and other components are mounted.
An equivalent circuit thereof is shown in FIG. 1B. a plurality of LEDs (LED-1, LED-2, . . . , LED-6) as a first group are serially connected between a power supply (voltage) line (+24V) 101 and ground line (GND) 102, a power source circuit IC101 is connected to an output of the LED-6, and an LED-7 and a resistance R101 are serially connected between an output of the power source circuit IC101 and the GND. In the same way, LEDs (LED-1A, LED-2A, . . . , LED-6A) as a next group are serially connected, an output of the LED-6A is connected to a power source circuit IC102, and an LED7A and a resistance R102 are serially connected between an output of the power source circuit IC102 and the GND. Such connection is considered as one group and arranged repeatedly.
As explained above, a display unit of a display device composed of LEDs arranged repeatedly on a substrate is cut to be any length for use in accordance with need. However, dirt, etc. may adhere to a cut surface of the substrate and cause a leak current in some cases.
Also, for example, dusts may adhere to both terminals of the LEDs and resistances and cause a tracking phenomenon, which results in leakage. Furthermore, when a high current flows, an electromigration, wherein wires are short-circuited, may be caused to generate leakage and, in the extreme case, short-circuiting results in a high current flow.
Other than the leak currents as explained above, leak currents of the LEDs themselves also arise. Electric characteristics of an output current (a forward current) with respect to an application voltage (forward voltage) of an LED are shown in FIG. 2A and FIG. 2B.
FIG. 2B shows forward bias characteristics of each LED. In a graph in FIG. 2B, the abscissa axis indicates a forward voltage from 1.0V to 6.0V and the ordinate axis indicates a forward current in a log-scale in a range of 0.1 mA to 100 mA. A red LED exhibits a forward current of 5 mA when a forward voltage is about 2.0V, and 0.001 mA or lower when 1V at the room temperature (25° C.). A green LED exhibits a forward current of 2 mA when a forward voltage is about 3.0V, and 0.001 mA or lower when 1V, and a blue LED exhibits a forward current of 2.0 mA when a forward voltage is about 3.0V, and 0.001 mA or lower when 1V.
From the data, a blue LED and red LED exhibit a forward current of 0.001 mA or lower even when a forward voltage of, for example, 2.0 V is applied, so that it can be deemed that a current virtually does not flow.
Accordingly, in a lighting device using an LED, it is possible to detect a leak current excepting that in the LED under a condition, under which a current flowing in the LED can be ignored, by supplying an equal voltage to or lower voltage than the voltage applied to the LED.
Generally, a current limit has been set in the supply power source to prevent a leakage in a lighting device or an LED lighting device, however, a high current has to flow for lighting and it has been also the same in the case of an LED. When a high current flows to an LED, even if a leakage (a leak current) arises, as explained above, it is often the case that the leak current cannot be detected only by a current limit because the leak current is very small.
Furthermore, as a result that a current flows to the leakage side and a supply to the LED reduces, a change of the current due to the leakage may be not enough to be detected comparing with the normal change. Therefore, it has been desired to detect a leakage by other method than the current limiting method for preventing it.
As explained above, a leakage preventing device of the related art reduced an abnormal current and an excessive current by detecting an abnormal current being in proportional with a leak current between a power source and a load or by detecting an excessive current when it flows to the load.